The measurement of a revolution rate, which is required in the technology on many machines and plants, is physically equivalent to the measurement of the angular speed. It is generally known to measure the revolution rate of shafts by applying a periodic pattern to the circumference of the shaft, the pattern being known as an “encoder”, e.g. a toothed wheel, which is sensed by a sensor fixedly disposed adjacent to the shaft. The sensor is able to differentiate between a tooth and a tooth gap or other periodically changing properties, such as e.g. a magnetic field direction or an optical transparency. The sensor subsequently generates an output signal having the same periodicity as the sensed pattern.
Such a sensor outputs different signals depending on the available signal processing. These may be approximately sinusoidal signals that are mainly generated directly by the primary sensor element of the sensor, or square wave signals that are mainly generated by the downstream signal processing by means of comparators.
If in addition a direction detection of the motion observed by the sensor is necessary, the use is known of a separate additional system comprising another encoder and sensor element, which is shifted by a quarter of the periodic length of the pattern for the same structure. The direction detection is based on the phase shift of the signals between the two systems, which is always a quarter of the periodic length in magnitude but which changes between leading and trailing depending on the direction.
Alternatively, a direction-sensitive phase shift between two signals can be generated by two sensor elements that interact with the same encoder but that have a separation in the direction of motion that causes a suitable phase shift. This design is advantageous because of saving the second encoder. In particular, with a small separation of the two sensor elements it is additionally advantageous that the elements can be integrated, e.g. within a common housing and in the case of semiconductor elements may even be monolithically integrated.
The technique presented is used in the same way for the speed measurement of linear motions, wherein linear instead of ring encoders are used.
The term “sensor” used here includes the actual sensor element, which together with the encoder generates the signal, and the associated signal processing/evaluation unit.
A device of the type described above, which comprises a sensor with two sensor elements, is known. Such a device thus comprises an additional system that consists of a second sensor element and the associated signal path (amplifier, filter etc.). Said additional system leads to associated costs. Also the selection of a fixed separation of the sensor elements is disadvantageous because the use of encoders with different periodic lengths is restricted. Indeed it is not necessary for the phase shift to be a quarter of the periodic length of the pattern, but said phase shift results in the highest signal magnitude with regard to the determination of the directions of motion. In the case of small values the requirements on the measurements of the phase shift are increased. However, providing an adapted separation of the sensor elements for each encoder or each application acts a cost driver as a result of the small numbers of items for each of said systems. In particular, the monolithic integration of semiconductor elements, such as e.g. Hall generators, is changed from a cost advantage to a disadvantage in the case of small numbers of items as a result of application-dependent design of the relevant system.